Communications spacecraft are widely used to provide wide-bandwidth communications. Such spacecraft are extremely expensive, and as a consequence it is economically imperative to derive the maximum possible operating life. Communication spacecraft must maintain their antennas directed toward the regions which they serve. The environment in space is such that forces act on the spacecraft which tend to change its attitude, which in turn tends to direct the communications antenna away from the desired region. Attitude errors are corrected by attitude control systems, which use various techniques for generating the attitude-control moments. Magnetic torquers can be used, but tend to have a limited range of torque. Reaction or momentum wheels can be used to provide torque, and rockets or thrusters arranged to provide moments may also be used. When reaction or momentum wheels are used, secular increases in wheel speed can ultimately lead to destruction of the wheel due to centrifugal forces; the wheel speed is therefore periodically reduced by use of thrusters. Thus, attitude control of spacecraft, for the most part, requires the use of thrusters of some sort, such as chemical bipropellant or monopropellant, arcjets, or the like. Such thrusters require propellant. In some orbits, propellant must also be used to maintain the spacecraft at the proper station. The end of the useful life of a spacecraft, then, in the absence of a malfunction, occurs when the supply of propellant is exhausted. Every effort is therefore made during the design and construction of the spacecraft to minimize the weight of the structure and payload, so that more propellant can be carried into orbit.
As mentioned above, a communication spacecraft ordinarily carries at least one antenna, by which communication is established with appropriate terminals on the Earth. In the past, reflector antennas have been used, with one or more horns located at a feed point of the reflector. Because of the difficulties of achieving the desired beam shape with a reflector-type antenna, and of reconfiguring the beamshape as conditions require, attention has been directed to array antennas, which can be remotely reconfigured by adjustment of phase shifters and/or level controls associated with each antenna element of the array. Reliability and considerations of the weight of beamforming networks has pushed the art toward the use of active array antennas, in which each antenna element of the antenna array is associated with at least an active element in the form of an amplifier, either for transmission or for reception, or both. The art of array antennas, and of active array antennas, is well advanced.
The use of active array antennas on spacecraft is not without its problems. At the current state of the art, the individual amplifiers which are associated with each antenna element for transmission tend, taken as a whole, to be less efficient than the prior-art amplifiers used in central-feed-with-beamformer antennas, so more electrical energy is required to operate an active array antenna than a central-feed array. The heat generated by the active elements of an active array antenna must somehow be removed from the antenna. In the case of active array antennas in the form of one or more thin panels, heat removal may often be accomplished by arranging the side of panel, opposite to the side with the antenna array, as a heat radiator, which allows heat to be rejected to space. Another problem is that of providing the increased electrical energy required to provide a given level of transmitted power. The power problem can be solved only by providing a suitable power source. Nuclear power sources cannot be justified, so a sufficiently large or efficient solar array must be provided for converting sunlight into electrical energy. The electrical energy can be used for spacecraft housekeeping as well as for energizing the communications payload.
Another problem with active array antennas is their size. The antenna gain is related to the surface area of the antenna, and achieving the high gain needed for the desired beam contour control requires a large active surface. At the time of launch of the spacecraft, the array antenna and the solar panels must be fitted, together with the main body of the spacecraft, into the shroud of a booster rocket. This constraint requires that the array antenna and the solar array be arranged in the form of stowable panels, which are folded or otherwise collapsed against the body of the spacecraft to make a structure which is small enough to fit into the shroud. This requirement, in turn, leads to a requirement for a mechanism to deploy the solar panels to form the solar array, and to deploy the active array antenna. This problem is exacerbated by the large forces acting on the stowed panels during launch of the booster vehicle. While it is easy in concept to provide a strong deployment mechanism, it is difficult to do so within the weight and reliability constraints of a spacecraft.
Ordinarily, spacecraft are not launched to their final orbits in a single maneuver. Instead, the booster vehicle boosts the spacecraft to an intermediate or transfer orbit. After some time, the intermediate orbit may be adjusted by the use of on-board thrusters, and then a further velocity change maneuver is performed to boost the spacecraft to the final orbit. The stowed antenna and solar array panels remain in the stowed positions until the spacecraft is in the final orbit, to prevent damage due to the forces attributable to the velocity change maneuver. While in the intermediate orbit, the spacecraft is not fully operational, but is subject to many of the same environmental conditions as those in its final orbit.
During normal operation of the antenna array, the antenna array panels are deployed, and the active elements associated therewith are energized. As mentioned above, the waste heat resulting from the operation of the active elements of the active antenna array tends to heat the antenna panels. The heat-rejecting side of the antenna panels face empty space, and radiate heat to tend to maintain the temperature of the active elements of the active antenna array within design temperature limits selected for reliable long life. Such a temperature range might be, for example, -60.degree. C. to +120.degree. C. During the transfer orbit, however, the active elements of the antenna array are not energized. The heat-rejecting side of each panel, if exposed to space, would continue to reject heat, and might possibly result in a panel temperature less than the design minimum.
Also during normal operation of the solar array, the array panels are deployed, and a mechanism orients the active side of the solar panels toward the sun, so that maximum electrical power is always available, except when the spacecraft is in eclipse. During the intermediate orbit, however, the solar array panels are not deployed, and little electrical power may be available. Some electrical energy may be required to heat the antenna array panels, to offset radiation from the panels, to prevent their temperatures from dropping too low. If the batteries of the spacecraft are used to provide electrical energy for heating the active antenna array, the duration of the intermediate orbit cannot exceed the length of time which the battery charge can sustain, which places undesirable constrains on operation of the spacecraft. As an alternative or supplement to the use of heaters, the active antenna array panels may be insulated to prevent radiation to space, and thereby substantially reduce the electrical heater power required. However, the insulation has undesired mass, and may tend to interfere with the deployment of the active antenna array panels.
Improved spacecraft arrangements are desired.